The goal of this proposal is to explain macroscopic behavior of locomoting cells based on biochemical and biophysical properties of actin filaments and actin-associated proteins. Amoeboid cell motility underlies many normal and pathological processes including morphogenetic movements during embryonic development, movement of axons and dendrites during remodeling of the nervous system, chemotactic movement of immune cells, migration of fibroblasts during wound healing, and dispersal of metastatic tumor cells. Amoeboid motility requires a set of six essential proteins - actin, the Arp2/3 complex, WASP-family proteins, capping protein, cofilin, and profilin - whose structures and functions are conserved across eukaryotic phyla. Together these proteins form a biochemical module that constructs dynamic, force-generating networks of actin filaments in almost all eukaryotes. Understanding the function of this biochemical module will give us insight into motility of normal cells as well as the aberrant motility of pathological cells including migratory metastatic tumor cells. To generate a mathematical description of cell motility we perform quantitative studies at three size scales: (i) the biophysical properties and interactions of actin-associated proteins, (ii) the mechanics of actin-based motility and force generation in vitro, and (iii) the control of the actin assembly machinery by signaling systems in intact cells. In pursuit of our goal we ask the following questions: 1. What is the mechanism by which WASP-family proteins stimulate nucleation activity of the Arp2/3 complex? 2. Apart from activating the Arp2/3 complex, how do WASP-family proteins promote actin filament assembly? 3. How do the kinetics of filament assembly and crosslinking control the architecture of motile actin networks? 4. How do signaling systems work to control the actin-based motility module in vivo?